The current optical communication systems are high level and complicated systems, such as wavelength multiplexing (WDM) communication using dual polarization quadrature phase shift keying (DP-QPSK) phase-modulated signals, in order to deal with an increase in the transmission capacity. Therefore, high-performance semiconductor optical elements are required.
One of the most effective means for realizing semiconductor optical elements with advanced performance is a monolithic integration that is forming a number of semiconductor optical elements in one chip. For example, modulator integrated semiconductor lasers are known where a semiconductor laser and an electro-absorption semiconductor optical modulator (EA modulator) of a compound semiconductor material are integrated (see Patent Document 1). SOA integrated semiconductor lasers are also known where a semiconductor laser and a semiconductor optical amplifier (SOA) are combined (see Patent Document 2). These integrated semiconductor optical elements have a number of functions such as laser oscillation and optical modulation or laser oscillation and optical amplification in one chip, and therefore, it is possible to implement a compact semiconductor optical element with advanced performance.
In addition, Mach-Zehnder (MZ) type modulators have been proposed as modulators using a compound semiconductor, and capacitor-loaded MZ modulators have been proposed as MZ modulators with particularly advanced performance (see Patent Document 3). In capacitor-loaded MZ modulators, an electrode is formed so as to be divided into portions along a waveguide, and it is possible to adjust the impedance by varying the ratio of the electrode portions. Thus, an element structure matched to 50Ω can be easily implemented, which makes it possible to obtain high radiofrequency properties.
In order to realize a stable operation in such integrated semiconductor optical elements, it is necessary for the divided electrodes to be sufficiently isolated electrically. In order to do so, such a structure of a capacitor-loaded MZ modulator has been proposed where the clad layer on top of the waveguide is a p type InP layer directly beneath the electrode portions and is an i type InP layer in the isolation portions between the electrode portions. In this structure, the clad layer between the portions of the electrode formed so as to be divided along the waveguide is an i type semiconductor layer having high resistance, which prevents leakage between the electrode portions through the clad layer, and therefore, a stable operation becomes possible.
In accordance with a general technique that is used in order to make some portions of the clad layer on top of the core layer of the above-described waveguide a p type InP layer and the other portions an i type InP layer, a p type InP clad layer is grown once on the entire surface, and after that, some portions are removed, and then, an i type InP layer is regrown in these portions. In this method, it is easy to control the doping of impurities into both the p type InP layer and the i type InP layer, which makes it possible to form the clad layer that is strictly divided into i type and p type portions.
Here, a conventional capacitor-loaded MZ type modulator is described in reference to FIGS. 12A and 12B. FIGS. 12A and 12B illustrate a conventional capacitor-loaded MZ type modulator, where FIG. 12A is a plan diagram and FIG. 12B is a cross-sectional diagram along the waveguide. As illustrated in FIG. 12A, the waveguide is provided with two waveguide arms between an input waveguide 75 and an output waveguide 76 so that light 77 that has been inputted is branched into the two waveguide arms. As illustrated in FIG. 12B, the waveguide has such a structure where an n type InP clad layer 62 and an InGaAsP core layer 63 are provided on top of a semi-insulating InP substrate 61, and on top of that, i type InP clad layer portions 67 and p type InP clad layer portions 64 are provided so as to alternate in the layer. Here, a p type InGaAsP contact layer 65 is provided on top of the p type InP clad layer portions 64 if necessary.
The space between the waveguide arms that have been etched in stripe form is filled in with an embedded insulating layer 69. An electrode on top of the waveguide 72 is selectively provided on the p type InGaAsP contact layer 65, and the portions of the electrode on top of the waveguide 72 are brought together for each waveguide arm so as to be connected to a wide electrode 70 and 71 respectively. A high frequency signal source 73 is connected between the wide electrode 70 and the wide electrode 71 on the input side, and a terminal resistor 74 of 50Ω is connected between the wide electrode 70 and the wide electrode 71 on the output side.
The light signal that has entered through the input waveguide 75 is branched into the two waveguide arms that form a modulation waveguide, modulated by a high frequency signal 78 that has been applied by a high frequency signal source 73, and is outputted from the output waveguide 76 as modulated light 79.